The invention relates to matrix display systems, particularly video display systems, for example, for displaying TV pictures, and methods of operating such systems.
The invention concerns matrix display systems, for example, liquid crystal display systems, of the kind comprising a display panel having a row and column array of picture elements for modulating light, means for illuminating the display panel, and drive means for driving the picture elements in accordance with an applied video signal, the picture elements being driven a row at a time by scanning the rows in sequence repetitively in respective field periods.
Video display systems comprising display panels having light modulating picture elements, such as liquid crystal elements, for displaying TV pictures and the like are well known. Larger area display panels commonly include an active switching device, for example a TFT or thin film diode, associated with each picture element to improve display quality. In operation, the display panel is continuously illuminated by a light source and the picture elements serve to modulate the light according to applied video information to produce a display output. The picture elements are connected to row and column address conductors and are driven one row at a time in sequence by scanning the row conductors with selection signals so as to transfer video information, data, signals, derived by sampling an input video signal, on the column conductors to the respective picture elements. After all rows have been addressed in one field period, the operation is repeated with each row being addressed in successive field periods. In these known systems, the scanning, and hence field, frequency is determined by timing signals of the input video signal and the field rate of the display panel corresponds to that of the supplied video signal. In the case of a PAL TV, for example, each row of picture element is addressed in a line period of 64 microseconds or in the interval between successive line periods, which occurs once every 20 milliseconds, corresponding to the field period. In the case particularly of active matrix display devices, then for the interval between successive addressing (corresponding to a field period) the picture elements are effectively isolated so that charge is stored on the picture element and the display effect produced by the picture element is substantially maintained until the element is next addressed in the subsequent field period.
The use of active matrix addressing, using for example TFTs or MIMs, and twisted nematic liquid crystal materials has enabled many of the requirements demanded for video displays, for example grey scales, contrast and brightness, to be satisfied to an extent. For acceptable viewing, the display system should be capable of providing good display quality for rapidly moving images as found in TV or computer generated graphics. There is a need for improvement in this respect, especially with regard to blurring or smearing effects which occur when displaying moving images. The effect tends to be particularly noticeable for bright objects moving against a dark background.
It has been found that the time taken for the transmission of a picture element to stabilise after a change in the drive level can be significant and in a typical display panel it may take several fields for a picture element to settle to its now transmission level when the drive level is changed from, for example, a 90% transmission to 10% transmission and with a panel being driven at a field rate of 50 Hz corresponding to the field rate of an applied video (PAL) signal. There is described in EP-A-0487140 a method of driving an active matrix liquid crystal video display device which leads to an improvement in this respect. In this method, the picture elements are driven at a field rate which is higher than that of the applied video signal. For example, in the case of the applied video signal comprising a PAL or NTSC TV broadcast signal whose field rates are 50 Hz and 60 Hz respectively, the field rate for the display panel may be increased to 100 Hz and 120 Hz respectively. This has been found to reduce significantly the time taken for the transmission of a picture element to stabilise after a change in the drive (video) level.
Whilst this approach provides a significant improvement with regard to smearing effects, it has been found that certain unwanted visual effects in the form of a perceived lack of sharpness or blurring, particularly at the edge of moving objects, can still occur.
It is an object of the present invention to provide a matrix display system which offers improved display quality when displaying moving images and a method of operating a matrix display system which helps to alleviate the problem of unwanted visual effects when displaying moving images.